Frequency variation response circuit



y 15, 1940- I. zAKARlAs 2,208,091

FREQUENCY VARIATION RESPONSE CIRCUIT Filed June 5, 1957 PLHTE CURRENTFREQUENCY FREDUENCY f0 6 Fig-40 Fi9n2- AxourPgr AF. bu rpur 8 l-LEINPUT2 I a owc ifafiu pick-140 Fig.3. Fig.4

INVENTOR- ATTORNEYS Patented July 16, 1940 FREQUENCY VARIATION RESPONSECIRCUIT Imre Zakarias, Budapest, Hungary, assignor to Radio PatentsCorporation, New York, N. Y., a corporation of New York Application June3, 1937, Serial No. 146,180 In Hungary June 10, 1936 14 Claims.

The present invention relates to means of and a method for convertingfrequency variations including both the frequency of an alternating oroscillating current and the natural or tuning frequency of a resonantcircuit into corresponding amplitude or intensity variations of anelectric current or potential.

In practice, the problem arises in many cases to convert frequencychanges into corresponding intensity changes of a current or voltage. An

example is the case of the electrostatic or condenser microphone havinga diaphragm vibrated in accordance with air pressure variations producedby speech or music and causing corre- 18 sponding variations of itselectrical capacity. In order to convert the capacity variations of sucha microphone it has been proposed to connect the same in a resonantcircuit excited by a high frequency potential having a frequencyslightly dif- 30 ferent from the natural frequency of the circuit in thenormal or unmodulated condition of the microphone or in other words tooperate the circuit on the steep portion of its resonancecharacteristic. Thus, capacity variations of the ml- 35 crophoneproduced by speech or music cause corresponding potential variations atthe terminals of the resonant circuit. If the latter are applied to anelectronic device or the like operating as a rectifier, a rectifiedoutput current fluctuating 30 according to the original capacityvariations or in turn the vibrations of the microphone diaphragm isobtained.

An object of the present invention is the provision of an improvedmethod of and system for 35 converting variations of the frequency of anoscillating current or of the natural or tuning frequency of a resonantcircuit into corresponding variations of amplitude or intensity of aunidirectional current or potential requiring only 40 a limited numberof circuit elements compared with arrangements and methods known in theprior art.

Another object is to provide a conversion system for changing varyingfrequencies into corre- 5 sponding variations of amplitude characterizedby greatly increased sensitivity and freedom from noise and otherinterference compared with prior art arrangements.

A more specific object is to convert variations 50 of electric circuitconstants determining the tuning characteristics of a resonant circuitinto corresponding variations of amplitude or intensity of an electriccurrent or potential.

Still another object is to convert small variaw tions of electriccapacity into corresponding relatively large variations of a directcurrent or potential.

A further object is to provide a discriminator circuit for producing avarying direct current or potential in accordance with the degree of de-5 tuning of a resonant circuit relative to an alterhating potentialimpressed thereon.

The above and further objects of the invention will become more apparentfrom the following detailed description taken with reference to 10 theaccompanying drawing forming part of this specification and wherein:

Figures 1 and 2 are characteristic curves explanatory of the functionand operation of an electron valve conversion system for transform- 1'ing variations of frequency into corresponding variations of amplitudeor intensity of an electric current or potential,

Figure 3 shows a basic circuit arrangement for practicing the invention,and 20 Figure 4 shows a modification of a circuit according to theinvention.

With the above objects in view, the invention is based on what ishereafter referred to as the space charge coupling effect inmulti-electrode electronic devices. This eifect may be explained by thefollowing. Considering a four-element electron valve or tetrode ofstandard type comprising a cathode, a first or control grid, a second orscreen grid electrode and a plate or anode and assuming further that thescreen grid is biased positively in a known manner and that the plate,contrary to known practice, is given a zero or negative bias relative tothe cathode,

a space charge cloud or concentration of elec- 5 trons also known as avirtual cathode will be formed between the screen grid and the plate dueto the deceleration or slowing down of the electrons passing the meshesof the screen grid by the negative or zero potential on the plate. 40This space charge will vary in accordance with s the variations of theelectron current, that is in accordance with the potential impressedupon the control grid. and if the latter is a high frequency potentialthe space charge will oscillate at the same frequency and induce byelectrostatic coupling a corresponding potential upon the plate. As aresult a current will be set up in the plate circuit and correspondingvoltage variations obtained at the terminals of a load impedanceincluded in the plate circuit.

In a similar manner a high frequency potential is induced upon thefourth grid usually the input control grid of a pentagrid converter ofstandard construction wherein the first and sec- 56 to generate a highfrequency oscillation. In this case, the voltage induced upon the fourthgrid through the space charge coupling has the same a frequency as theoscillator frequency. Since the current is induced capacitatively itleads the space current potential controlling the same on the first gridby 90.

If according to the present invention in a pentagrid valve of theaforedescribed type a resonant circuit is connected to the fourth gridelectrode which circuit is tuned to a frequency equal to the frequencyof the induced displacement current, the voltage and current in thiscircuit will be in phase or in other words the circuit will represent apure resistance to the electron discharge current. As a result, theinduced voltage leads the controlling voltage by 90 thereby exerting noinfluence on the plate or output current of the valve. If the resonantcircuit connected to the fourth grid is detuned relative to the inducingspace charge frequency the induced displacement current will lead thecontrolling voltage by an angle greater or smaller than 90 dependent onthe sense of detuning. As a result, a component of the induced voltagewill be in phase with or in counterphase to the inducing voltage in sucha manner that the average plate current is increased or decreasedaccordingly.

Referring to Figures 1 and 2 there are shown characteristic curves of afrequency converter constructed and operated in the above describedmanner. In Figure 1, curve A shows the output or plate current as afunction of the frequency of the potential impressed upon the firstcontrol grid while curve B of Figure 2 shows the corresponding variationof the screen grid (third and fifth grid in a pentagrid valve) currentf0 representing the natural or tuning frequency of the resonant circuit.Curves A and B correspond to a low damping of the resonant circuitconnected to the fourth grid while curves A and B correspond to a highdamping of the resonant circult. The latter is preferably a paralleltuned circuit such as shown in Figures 3 and 4 to be describedpresently. a

Practical experiments have shown that by'impressing a signal voltagehaving a frequency of about 1000 kc.upon the first control grid andby-providing a resonant circuit connected to the fourth grid having anaverage Q value, a detuning of the circuit by about 30 kc. resulted in achange of the plate current of about 3 ma. A'

detuning of 1% resulted in a change of 1 ma. Thus, by providing a plateload impedance of .3 megohm, voltage variations of the order of 300volts are obtained making it possible to use the system for controllingconsiderable energies.

For further discussion, a converting or mixing tube of simplifiedconstruction will be considered comprising a pair of control grids, oneof which serves for applying a signal or oscillating potential, whilethe other grid has connected to it a resonant circuit in the mannerdescribed hereinabove. The first control grid is usually placed next tothe cathode and preferably a further positively biased screen grid isprovided between the two control grids.

a space charge or virtual cathode is produced in the space adjacent tothe second control grid. If desired, the latter may be further screenedfrom the anode and an additional suppressor grid provided in a mannerwell known.

By biasing both control grids negatively relative to the cathode ondgrids are connected to an oscillator system As seen from Figures 1 and2, the damping of the resonant circuit has an influence on both themagnitude of the plate direct current changes as well as on thefrequency band width which can be'transmitted and demodulatedfaithfully. It is furthermore possible to utilize the effect of variabledamping on the output current similar to the utilization of the changesof detuning of a resonant circuit or of the impressed frequencyvariations as described hereinabove. If the effect of variable dampingis utilized, it is advisable to operate the tube at a point of maximumplate current, point a as indicated in Figure 1.

. The resonant circuit connected, to the second grid may also be in theform of a series tuned circuit in place of a parallel tuned circuitshown in Figures 3 and 4. It may furthermore be an aperiodic circuit andcomprise capacity and resistance elements only, in which latter case thecircuit inductance is formed by the wiring or connecting leads. In thiscase, it is preferable to operate the tube not within the central orstraight line portion w-b of its characteristic but rather within one ofthe outer branches. These branches are similar to the grid voltage-platecurrent characteristic of a variable mu type electron valve of thepentcde type. This operation is advantageous if the frequency-amplitudeconversion is to be carried out in accordance with a non-linear such asa logarithmic function. A further characteristic of the outer branchesis the fact that one is below and the other above the normal platecurrent by which is understood the plate current flowing through thetube if the second control grid is grounded with regard to highfrequency. This feature of the characteristic may be utilized todetermine whether a frequency applied to the first control grid issmaller or greater than the frequency applied to the second control gridas is often desirable in carrying out inductance, capacitance orfrequency measurements by meansof a system of the type of the invention.

The plate direct current of a multi-grid converter valve of the typedescribed varies in accordance with the phase difference between the ivoltages applied to the different grids either directly or by spacecharge coupling as explained above thereby enabling such a valve to beemployed as a power measuring device.

In employing a system of 'the aforesaid type embodying a resonantcircuit connected to the second control grid and means for impressing apotential upon the first control grid having a frequency near thenatural frequency of said resonant circuit, the voltage induced in saidresonant circuit through space charge coupling may attain extremelylarge values particularly in case of short waves in such a manner thatif the second control grid is not sufficiently negatively biased a gridcurrent will start to flow. This grid current constitutes an additionaldamping of the resonant circuit whenever the amplitude of the highfrequency potential exceeds the normal or steady biasing potential. Thisadditional damping results in an improvement of the linear portion ofthe mid-section a--b of the average plate element is subject to slowfluctuations caused by external influences such as in the case of acondenser microphone operating in open air whereby displacements may becaused by air currents or drafts, an extended linear portion of thecharacteristic will enable a stable and faithful operation irrespectiveof such variations.

According to a modified arrangement, the signal input grid and the gridhaving the resonant circuit connected to it may be mutually exchanged,that is the resonant circuit may be connected to the first control gridof a simplified converter valve of the type above mentioned and a highfrequency input potential impressed upon the second control grid. Inthis case the interelectrode capacity between the grids will produce thenecessary coupling especially in the case of short waves whereby thesame converting effect may be obtained as described and frequencychanges transformed into corresponding changes of amplitude of theaverage plate current. For longer waves the operation can be improved bythe addition of an external capacitative, inductive or galvaniccoupling. However, a pure elec tron coupling has proved to be superiorto other couplings for reasons similar to those well known in theoperation of frequency changing electronic converter or mixer valvesused in superheterodyne radio receivers.

The electron coupling effect for the purpose of the invention may alsobe secured in a pentagrid converter tube of standard construction byconnecting the resonant circuit to the second positively biased oroscillator grid and by coupling the circuit connected to this grid withthe circuit of the first grid. Since the second grid draws electronsfrom the space charge formed near the fourth grid a similar effect onthe output current is obtained as in the previously describedarrangements.

If the modulating frequencies are not to be translated uniformly in theplate circuit a frequency discriminating load impedance of desiredcharacteristic may be provided in the latter.

As is evident from the foregoing, the present invention contemplates theemployment of a multi-grid electronic discharge valve with means forimpressing a voltage of constant or variable frequency upon one of thegrid electrodes and a resonant circuit of constant or variable resonantfrequency connected to another grid electrode, whereby said resonantcircuit is excited by said impressed potential solely by coupling withthe electron discharge current through said valve.

Referring to Figure 3, there is shown a basic circuit of the typeaccording to the invention. This circuit among other uses may serve as areceiver or demodulator for frequency modulated signals. The demodulatorvalve 2 comprises a cathode 24 and anode of known construction, a firstor signal input grid I, a second control grid and a screen grid I4interposed between the former to prevent mutual interaction between thegrids. The received frequency modulated input signals which may bederived from an antenna or any other source of high frequency signalsconnected to the terminals designated H. F. input in the drawing andwhich may have been amplified by a preceding amplifier are impressedupon the first control electrode I and the cathode 24, of the valve 2through a grid coupling condenser 4 and a grid leak resistance 3 in amanner well understood. There is connected to the second controlelectrode or grid 5 and cathode 24 a'resonant impedance means such as atuned circuit comprising an inductance 6 shunted by an adjustablecondenser I and a variable resist- V ance O. In the example shown thecontrol electrode 5 is connected to a tap point of the inductonce 6 nearthe high potential side of the resonant circuit 6, I, 8 to ensure mostfavorable operation of the system. The natural or tuning frequency ofthis circuit is adjusted so as to be near the carrier frequency or nearone of the side-band frequencies of the impressed input signal. Theresistance 8 serves to adjust the damping and operating characteristicof the circuit. It is also possible to vary the tuning or naturalfrequency of the resonant circuit according to a prearranged schedule insynchronism with corresponding variations of the transmitting frequencyfor secret communication purposes. The circuit GI is further designed toaccommodate the desired modulation band width for which purpose it maybe constructed as a band-pass circuit or have its damping suitablyadjusted by means of the resistance 8. The demodulated output voltage isderived from the plate circuit by the aid of a low frequency loadimpedance orresistance 9. The high tension plate supply source is shownat I ii, and the heating supply sourceat I I. The grid biasing potentialis provided in a known manner by means of a resistance I 2 inserted inthe cathode lead and by-passed by a condenser I 3. The screen grid l4surrounding the grid 5 is biased positively relative to the cathode byconnection in a known manner to the positive pole of the high tensionsource III through a resistance l5 by-passed by a condenser I6. The highfrequency currents are further by-passed in the plate circuit by acondenser I 1 while a further resistance I8 is provided in the outputcircuit to prevent regeneration of high frequencies. The demodulatedoutput signals obtained from terminals A. F. may be further amplifiedand impressed upon a translating or reproducing device such asaloudspeaker or the like.

A system of the type described may also serve as a discriminator forautomatic frequency control in a radio receiver. For the latter purpose,there is usually provided in connection with a superheterodyne receiveran error detector or discriminator system producing a steady potentialvarying in accordance with the degree of detuning of an incoming signalfrequency relative to the fixed intermediate frequency for which thereceiver is' designed and which serves to control a tune adjustingelement associated with the local or heterodyne oscillator of thereceiver, so as to control the intermediate signal frequency tocorrespond with the tuning frequency of the intermediate frequencyamplifier. In using a system according to Figure 3 as a discriminator ortune responsive circuit, a potential derived from the intermediateamplifier is impressed upon the grid I through terminals H. F. and theresonant circuit 6! tuned accurately to the intermediate frequency ofthe receiver. The variations of the average plate current oralternatively of the screen grid current (see Figures 1 and 2) as causedby a change of the frequency of the local oscillator or detuning of thereceiver are utilized in a known manner to compensate the frequencychange by readjusting the local oscillating frequency. The regulating ortune adjusting voltage in this case is supplied from-the terminals A-F.The regulating range can be adjusted by controlling the damping of theresonant circuit such as by varying the resistance 8 as is understoodfrom Figures 1 and 2, or alternatively a second intermediate frequencymay be produced by double heterodyning to obtain the same purpose.Automatic frequency control can also be used in cases where the localoscillator frequency of the superheterodyne receiver is subject tovariations or drift caused by external influences or for secretcommunication purposes.

The arrangement of Figure 3 may also be used for controlling orstabilizing the frequency of a transmitter in which case the resonantcircuit 8-1 may contain a piezo crystal having a frequency correspondingto the transmitting frequency. In this case the plate current variationsmay serve to control the frequency of the master oscillator of thetransmitter through mechanical, electro-magnctlc or pure electric meansin a manner well known. Furthermore, the transmitter may be frequencymodulated in a simple manner by applying frequency modulation to thecircuit li| such as by the provision of a condenser microphone in thecircuit provided that the frequency control is sufficiently free frominertia.

Another group of applications of the invention applies to all caseswhere the natural frequency of the resonant circuit is subject toperiodic or irregular variations caused by external influence on thecircuit constants such as the inductance, capacitance and resistancevalue of the circuit. In all these cases the tuning or natural frequencycan be kept constant by the provision of a variable circuit element suchas a plate condenser, one electrode of which has the form of a diaphragmof thin sheet metal vibratable by small variations of air pressuresimilar to a condenser microphone.

In this manner, mechanical displacements such as the movement of apick-up in a phonograph, the strings of musical instruments, etc., canbe transformed into electrical variations through a variable platecondenser of this type.

The invention has further use in connection with safety, alarm andsignalling systems, counting and Sorting arrangements wherein variablecapacities produced by the movements, etc., to be controlled may beconverted in the manner described. Other details such as amplifiers,both before and after the converting system, etc., are obvious.

Referring to Figure 4, there is illustrated a modification of theinvention for converting capacity variations such as those produced by acondenser microphone or other electrostatic pick-up into correspondingdirect current variations. In this embodiment there is shown a valve 20of the pentagrid type having a first grid I and a second positivelybiased grid l9 serving as an oscillator grid to produce a local highfrequency oscillation in a manner well known. For the latter purpose thecapacitative pick-up 2! serves as the tuning capacity of an oscillatoryor tank circuit further comprising an induction coil 22 in parallel tothe microphone 2| and connected to the grid I through a couplingcondenser 4 and a grid leak 3. In order to maintain sustainedoscillations in the circuit 2|, 22 the output circuit of the grid l9includes a feedback inductance 23 coupled with the inductance 22 of thetank circuit. The third and fifth grid are biased positively in theconventional manner. The resonant circuit 6, I, 8 is tuned close to theoscillator frequency and connected to the grid and cathode 24 in amanner previously described. As a result, average plate currentfluctuations caused by variations of the oscillator frequency inaccordance with the vibrations of the pick-up 2| are obtained across theload impedance 9 at output terminals A. F. The system may be modifled byusing an oscillator tank circuit of constant frequency (fixed condenser2|) ,and by replacing the condenser 1 of the resonant circuit by acondenser microphone or the like as is readily understood from theabove.

It will be evident from the above that the invention is not limited tothe specific arrangements and methods described herein for illustrationand that the underlying inventive thought and basic principle aresusceptible of numerous variations and modifications coming within thebroader scope and spirit of the invention as defined in the appendedclaims.

I claim:

1. A system of the character described comprising an electronic devicehaving means for producing an electron space current, a pair of controlgrids disposed at different points in the path of said space current.means for impressing an input potential of varying frequency upon one ofsaid grids, a resonant circuit connected to the other grid, meanswhereby said resonant circuit is excited in accordance with saidimpressed potential by electron coupling with said electron spacecurrent, an output circuit for said device, and means for derivingoutput energy from said device, said output energy varying in amplitudeproportionately to the frequency departure of the impressed inputpotential from the resonant frequency to which said resonant circuit istuned.

2. A system. of the character described comprising an electronic devicehaving means for producing an electron space current, a pair of controlgrids disposed at different points in the path of said space current,means for impressing an alternating potential of substantially constantfrequency upon one of said grids, a resonant circuit connected to saidother grid having a natural frequency normally equal to the frequency ofsaid impressed potential, means for controlling the resonant frequencyof said resonant circuit, an output circuit for said device, and animpedance in said output circuit being effective in building up anoutput potential varying in amplitude proportionately to the frequencydeparture of the impressed input potential from the resonant frequencyto which the resonant circuit is tuned.

3. A system of the character described comprising an electronic devicehaving a cathode and an anode, a first control grid near said cathode, asecond control grid disposed in spaced relation to said first grid, afurther positively biased grid between said first and second grids toproduce a virtual cathode adjacent to said second grid, means forimpressing a high frequency potential of varying frequency upon saidfirst grid, a resonant circuit connected to said second grid, wherebysaid resonant circuit is excited by capacity coupling with said virtualcathode, an output circuit for said device, and impedance means in saidoutput circuit being effective in building up an output potentialvarying in amplitude proportionately to the frequency departure of theimpressed potential from the resonant frequency to which said resonantcircuit is tuned.

4. In a frequency variation response circuit, a source of alternatingpotential, a tuned circuit, the relative frequency of said source withrespect to the frequency to which said circuit is tuned being variable,an electron discharge tube comprising means for producing an'electronspace current and a pair of control electrodes located in the path ofsaid space current, means for impressing potential from said source uponthe first control electrode, means for connecting said tuned circuit tothe second control electrode, means to produce a concentrated electronspace charge near said second control electrode, the relation betweenthe frequency of said source and the frequency to which said circuit isresonant being such that said circuit is excited by coupling with saidspace charge at varying phase in accordance with the relative frequencydeparture between said source and the resonant frequency of saidcircuit, an output circuit for said tube, and load impedance means insaid output circuit adapted to develop output potential varying inamplitude proportionately to said frequency departure.

5. A system as claimed in claim 4 including means for controlling thedamping of said tuned circuit.

6. A system of the character described comprising an electronic devicehaving a cathode and an anode, a first control grid disposed near saidcathode, a second control grid disposed in spaced relation to said firstgrid, a further positively biased grid between said first and secondgrids to produce a virtual cathode near said second grid, means forimpressing a potential of substantially constant frequency upon saidfirst grid, a resonant circuit connected to said second control grid,said resonant circuit normally tuned to the frequency of said impressedpotential, means for controlling the resonant frequency of said resonantcircuit, an output circuit for said device, and load means in saidoutput circuit for deriving output energy, said output energy varying inamplitude proportionately to the frequency departure of the impressedpotential from the resonant frequency to which said resonant circuit istuned.

7. In a frequency variation response circuit, a source of alternatingpotential, a tuned circuit, the relative frequency of said source withrespect to the frequency to which said circuit is tuned being variable,an electron discharge tube comprising a cathode and an anode forproducing an electron space current and a pair of control grids disposedin the path of said space current, means for impressing potential fromsaid source upon the grid near the cathode, means for connecting saidtuned circuit to the other grid, means to produce a concentratedelectron space charge near said other grid, the relation between thefrequency of said source and the resonant frequency of said tunedcircuit being such that said tuned circuit is excited by said impressedpotential through electron coupling with said space charge at varyingphase in accordance with the relative frequency departure between theresonant frequency of said circuit and the frequency of said source, anoutput circuit for said tube, and load impedance means in said outputcircuit adapted to develop output potential varying in amplitudeproportionately to said frequency departure.

8. In a frequency variation response circuit, a source of alternatingpotential, a tuned circuit, the relative frequency of said source withrespect to the frequency to which said circuit is tuned being variable,an electron discharge tube comprising a cathode and an anode forproducing an electron space current and a pair of control grids disposedin the path of said space current, a screen grid located between saidcontrol grids, means for maintaining said screen grid at a positivepotential with respect to the cathode, means for impressing potentialfrom said source upon the control grid near the cathode, further meansfor connecting said tuned circuit to the other control grid, therelation between the frequency of said source and the resonant frequencyof said tuned circuit being such that said tuned circuit is excited bysaid impressed potential through electron coupling with said spacecurrent at varying phase in accordance with the relative frequencydeparture between the resonant frequency of said circuit and thefrequency of said source, an output circuit for said tube, and loadimpedance means in said output circuit adapted to develop outputpotential varying in amplitude proportionately to said frequencydeparture.

9. A system of the character described comprising an electronic devicehaving means for producing an electron space current, a pair of gridelectrodes disposed in one section of said space current, meanscomprising an oscillatory tank circuit and feedback means connected tosaid grids to cause sustained carrier oscillations of the electron spacecurrent, means for controlling the frequency of said oscillations inaccordance with variations of a modulating magnitude, a further griddisposed in a different section of said space current, a resonantcircuit connected to said further grid tuned to the carrier frequency,an output circuit for said device, and impedance means in said outputcircuit being effective in building up an output potential, said outputpotential varying in amplitude proportionately to the relative frequencydeparture of said oscillations from the carrier frequency.

10. A system of the character described comprising an electronic devicehaving a cathode and an anode, a pair of grid electrodes disposed nearsaid cathode, means comprising an oscillatory tank circuit and feedbackmeans connected to said grids to cause sustained carrier oscillations ofthe electron discharge current, means for controlling the frequency ofsaid oscillations in accordance with variations of a modulatingmagnitude, a control grid disposed in a different section of saiddischarge path, a positively biased grid connected between said firstmentioned grids and said control grid, a resonant circuit connected tosaid control grid tuned to the carrier frequency, an output circuit forsaid device, and impedance means in said output circuit being effectivein building up an output potential, said output potential varying inamplitude proportionately tothe frequency departure of said oscillationsfrom the carrier frequency.

11. A system of the character described comprising an electronic devicehaving a cathode and an anode, a pair of grid electrodes disposed nearsaid cathode, a regenerative system connected to said grids to causesustained oscillations of the electron discharge current between saidcathode and anode of substantially constant frequency, a control grid inthe path of said electron current, a further positively biased griddisposed between said first mentioned grids and said control grid, aparallel tuned resonant circuit connected to said control grid normallytuned to the frequency of said oscillations, means for controlling theresonant frequency of said resonant circuit, an output circuit for saiddevice, and impedance means in said output circuit being effective inbuilding up an output potential, said output potential varying inamplitude proportionately to the frequency departure of saidoscillations from the instantaneous resonant frequency of said resonantcircuit.

12. A frequency variation response circuit comresonant impedance means,the relative frequency of said source with respect to the frequency towhich-said impedance means :is resonant being variable, an electrondischarge tube comprising means for producing an electron space current,control means including circuit connections from said source to saidelectron-tube for varying said space current in accordance with saidsignal potential, a control electrode in said tube adapted to vary saidelectron space current in accordance with an electric potential appliedto it, means for connecting said resonant impedance means to saidcontrol electrode, further means for producing a concentrated electronspace charge adjacent to said control electrode, the relation betweenthe frequency of said source and the fre: quency to which said impedancemeans is res; onant being such that said control electrode is excited atvarying phase by said signal potential by coupling with said spacecharge in accordance with the relative frequency departure of saidsource from .the resonant frequency of said impedance means, an outputcircuit for said tube, and means operatively associated with said outputcircuitfor developing energy varying in amplitude in proportion to saidfrequency departure.

13. A frequency variation response circuit comprising an electrondischarge tube having means for producing-an electron space current, asource of alternating signal potential of varying frequency, controlmeans including circuit connections from said source to said electrontube for varying said space currentin accordance" with saidsignalpotential, a control grid in said tube, resonant impedance meansconnected to said controlgrid, means whereby said control grid isexcited by said signal potential substantially by electron coupling withsaid space" current, an output circuit for said tube, and meansoperative- 1y associatedwith said output circuit for developing energyvarying in amplitude proportionately to the frequency departure of saidsignal potential from the resonant frequency of said impedance means.

14. A frequency variation response circuit comprising an electrondischarge tube having means for producing an electron space current, asource of alternating potential of substantially constant fr uency,control means including circuit connec ions from said source to saidelectron tube for varying said space current in accordance with saidalternating potential, a control grid electrode in said tube, resonantimpedance means connected to said grid electrode having a resonantfrequency normally equal to the frequency of said source, means forcontrolling the resonant frequency of said impedance means, meanswhereby said control grid is excited by said signal potentialsubstantially by electron coupling with said space current, an outputcircuit for said tube, and

means operatively associated with said output circuit for developingenergy varying in amplitude proportionately to the departure of theresonant frequency of said impedance means from the frequency of saidsource.

IMRE ZAKARIAS.

